In a high frequency region including microwave frequencies and millimeter wave frequencies, dielectric ceramic materials have been used widely for high frequency circuit elements such as dielectric resonators, microwave integrated circuits, and the like. The dielectric ceramic materials for such elements are required to have a high Q value and a low temperature coefficient of resonant frequency. To this end, there have been proposed various dielectric ceramic compositions as a material for high frequency circuit elements.
For example, U.S. Pat. No. 4485180 discloses a dielectric ceramic composition consisting of Ba(Zr.sub.x Zn.sub.y Ta.sub.z)O.sub.7/2-x/2-3y/2, wherein 0.02.ltoreq.x.ltoreq.0.13, 0.28.ltoreq.y.ltoreq.0.33, 0.59.ltoreq.z.ltoreq.0.65, and x+y+z=1, and having a peak Q value of 9100 together with a temperature coefficient of resonant frequency of 1 ppm/.degree.C. at an operating frequency of 7 GHz.
Japanese patent laying-open No. 58-60661 discloses a dielectric ceramic composition consisting of Ba(Zn.sub.v Ni.sub.x Ta.sub.y Nb.sub.z)O.sub.7/2-3v/2-3x/2, wherein 0.03.ltoreq.v.ltoreq.0.33, 0.03.ltoreq.x.ltoreq.0.33, 0.ltoreq.y.ltoreq.0.70, 0.ltoreq.z.ltoreq.0.70, and v+x+y+z=1, and having a peak Q value of 10300 together with a temperature coefficient of resonant frequency of 0 ppm/.degree.C. at an operating frequency of 7 GHz.
These dielectric ceramic compositions can be used for circuit elements designed to operate at a high frequency of around 10 GHz and below but the disadvantage of such compositions is that they can not be used to produce microwave elements designed to operate at a frequency higher than 10 GHz since the Q value decreases with increase of an operating frequency. Recently, there is a tendency to operate electronic devices at higher frequencies, Thus, there is an increasing demand of dielectric ceramic compositions having a higher Q value together with a small temperature coefficient of resonant frequency.